Fixes so that a oneOf schema with a single sub-schema is simplified

[Mattias-Sehlstedt]

Fixes #21042

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[Mattias-Sehlstedt]

Would the project considered this as a breaking change due to how it might affect those that are currently using the incorrect single-oneOf behavior as an expected behavior? Or does a bugfix surpass that reasoning?

(If it is considered a breaking change, then I would love if this feature would be merged so that I could easily implement the fix locally)